docs: note about `react-native` <-> `react-native-mmkv` versions compativility

[vshkl]

I needed to use this library in a project with a rather old version of react-native (v0.65.3, for that matter). Of course, I defaulted to the latest version because the documentation didn't tell me otherwise. Only after some time I found out that in my case I can't use the latest version. Other people apparently sometimes find this information the hard way, as in this issue. So I decided it wouldn't hurt to make a note of it in the project README.

[tomgreco]

@mrousavy merge this my boia. It would've saved me days of time.